1. Field of the Invention
The present invention relates to a zoom lens, and more particularly, to a zoom lens which is preferable for use in an optical apparatus such as a video camera, a digital camera, a film camera, and a broadcasting camera.
2. Description of the Related Art
In recent years, a four unit zoom lens of a rear focus type is known as a zoom lens for use in a video camera, a digital camera, and an electronic still camera which use a solid-state image pickup element, and a camera which uses a film. The zoom lens is configured to have, in order from an object side, a first lens unit which has a positive refractive power and remains stationary during zooming and focusing, a second lens unit which has a negative refractive power and is movable during zooming to provide variable magnification, a third lens unit which has a positive refractive power and remains stationary during zooming and focusing, and a fourth lens unit which has a positive refractive power and is movable during zooming to compensate for variations of an image plane associated with varied magnification and is movable for focusing. Such a zoom lens has been proposed, for example, in Japanese Patent Application Laid-Open No. 1995-270684 (corresponding to U.S. Pat. No. 5,963,378), Japanese Patent Application Laid-Open No. 1995-318804 (corresponding to U.S. Pat. No. 5,963,378), Japanese Patent Application Laid-Open No. 1996-292369 (corresponding to U.S. Pat. No. 5,940,221), and Japanese Patent Application Laid-Open No. 1999-305124 (corresponding to U.S. Pat. No. 6,166,864).
In addition, a four unit zoom lens of a rear focus type which has an aspheric surface has been proposed in Japanese Patent Application Laid-Open No. 1993-72474 (corresponding to U.S. Pat. No. 5,235,466). In the zoom lens, a second lens unit is composed of a meniscus negative lens, a biconcave negative lens, and a positive lens. A third lens unit is composed of a single lens having one or more aspheric surfaces. A fourth lens unit is composed of a lens having one or more aspheric surfaces.
Attempts have been made to reduce the size of a third lens unit and a fourth lens unit which correspond to an image forming system in the four unit zoom lens of a rear focus type. For example, a known zoom lens has a third lens unit which is composed of a positive lens and a negative lens in order from an object and which is formed as a telephoto lens type by shifting the principal point of the third lens unit toward the object to reduce the actual distance between the third lens unit and the fourth lens unit to achieve a smaller size. Such a zoom lens has been proposed, for example, in Japanese Patent Application Laid-Open No. 1993-19165 (corresponding to U.S. Pat. No. 5,231,540), Japanese Patent Application Laid-Open No. 1993-297275 (corresponding to U.S. Pat. No. 5,396,367), Japanese Patent Application Laid-Open No. 1993-60973 (corresponding to U.S. Pat. No. 5,296,969), Japanese Patent Application Laid-Open No. 1993-60974, Japanese Patent Application Laid-Open No. 1993-107473, Japanese Patent Application Laid-Open No. 1994-130297 (corresponding to U.S. Pat. No. 5,396,367), Japanese Patent Application Laid-Open No. 1996-292369 (corresponding to U.S. Pat. No. 5,940,221), Japanese Patent Application Laid-Open No. 1996-304700, Japanese Patent Application Laid-Open No. 1999-305124 (corresponding to U.S. Pat. No. 6,166,864), Japanese Patent Application Laid-Open No. 2000-206407 (corresponding to U.S. Pat. No. 6,344,932), U.S. Pat. No. 5189558, and U.S. Pat. No. 5,189,558.
On the other hand, various vibration isolation optical systems (zoom lens) have been proposed which have a function of preventing an image blur (a vibration isolation function) caused when camera shake occurs due to vibrations transmitted to an image-taking system.
For example, Japanese Patent Application No. 7-128619 discloses a four unit zoom lens consisting of, in order from an object side, a first, second, third, and fourth lens unit having positive, negative, positive, and positive refractive power, respectively. In this zoom lens, the third lens unit comprises two lens units having positive and negative refractive powers in which the lens unit having the positive refractive power is caused to vibrate to correct an image blur. Japanese Patent Application No. 7-199124 (corresponding to U.S. Pat. No. 5,585,966) discloses a four unit zoom lens consisting of, in order from an object side, a first, second, third, and fourth lens unit having positive, negative, positive, and positive refractive power, respectively. In this zoom lens, the entire third lens unit is caused to vibrate to correct an image blur.
The present applicants have disclosed, in Japanese Patent Application No. 2001-42213 (corresponding to U.S. Pat. No. 5,414,800), a four unit zoom lens consisting of a first, second, third, and fourth lens units having positive, negative, positive, and positive refractive power, respectively, in which the entire third lens unit is caused to vibrate to correct an image blur.
In a zoom lens for use in a digital camera and a video camera, high optical performance yet compact design is needed in association with the trends toward a smaller size of the camera body and an increased number of pixels of an image pickup element in recent years. In addition, it is desirable for a video camera to record still images with high quality.
Typically, in a zoom lens, a higher refractive power of each lens unit results in a less amount of movement of each lens unit required to achieve a predetermined magnification ratio. Thus, high variable magnification and a reduced overall length of the lens can be provided.
Merely increasing the refractive power of each lens unit, however, causes larger variations in aberration during zooming or focusing. Higher accuracy is also needed in manufacture. For example, when lenses have relatively misaligned axes in a lens unit, the image forming capability is significantly degraded as compared with a zoom lens having a lower refractive power.
The rear focus scheme can provide advantages such as quick focusing while the size of the entire lens system is reduced.
On the other hand, high variable magnification is difficult to achieve, and large variations in aberration tend to occur in focusing. It is considerably difficult to provide high optical performance throughout the object distance from an object at infinity to an object at a short distance.
Especially when high variable magnification is intended, it is significantly difficult to provide high optical performance throughout its entire range of variable magnification and throughout the object distance.
It is an object of the present invention to provide a zoom lens which has a smaller size of its entire lens system and achieves high optical performance even with a high variable magnification ratio.
To achieve the aforementioned object, according to an aspect of the present invention, the zoom lens system comprises, in order from a front side (an object side in the case of an image-taking optical system) to a rear side (an image side in the case of an image-taking optical system), a first lens unit which has a positive power (power is the reciprocal of a focal length), a second lens unit which has a negative power, a third lens unit which has a positive power, and a fourth lens unit which has a positive power, wherein the second lens unit and the fourth lens unit are moved in an optical axis direction during zooming. The configuration of the second lens unit is specified, or the effective diameter of the first or second lens unit is specified, as described in embodiments described later.